Organosilazanes are compounds having the general formula R.sub.n Si(NH.sub.2).sub.4-n wherein R represents a monovalent hydrocarbon radical and n is 1, 2, or 3. The compounds can be obtained best by reacting corresponding organohalosilanes of formula R.sub.n SiX.sub.4-n, wherein R and n are as described above and X is a halogen atom, with ammonia, a process referred to as "ammonolysis." In the process, the halogen atoms are substituted in the first reaction step by NH.sub.2 groups, and then the halide-ions react with excess ammonia to form ammonium halide, which is immediately precipitated with a suitable solvent so that it does not take part in the reaction. Thus, two molecules of ammonia are required to replace one halogen atom.
The preparation of condensable silazanes by ammonolysis of the corresponding halogen silanes has been described by Nicholas D. Cheronis in U.S. Pat. Nos. 2,564,674, 2,579,416, and 2,579,418, all of which are incorporated herein by reference. According to these patents, ammonolysis was carried out with gaseous or liquified ammonia under atmospheric, or normal, pressure.
In ammonolysis with gaseous ammonia, the ammonia is passed, i.e., bubbled, through a solution of the organohalosilane. Depending on the temperature of the solution and the rate of introduction, considerable losses of ammonia are incurred. In addition, significant amounts of solvent are carried away with the ammonia. These losses are not only uneconomical, but they also lead to pollution of the environment.
These losses can be avoided to a great extent in ammonolysis with gaseous ammonia under normal pressure only if the reaction is carried out at temperatures far below the evaporation temperature of ammonia, -33.4.degree. C. Even with smaller batches, reaction temperatures of about -40.degree. to -60.degree. C. must be maintained to prevent sudden temperature increases, due to the introduction of excessive amounts of ammonia, and local temperature fluctuations. Beyond that, there is always the acute danger in ammonolysis with gaseous ammonia that the gas feed pipes will be clogged by ammonium halide deposits. Ammonolysis under normal pressure with preliquified ammonia has the disadvantage of raising cooling problems.
The reaction of organohalosilanes with liquid ammonia--whether the ammonia is pre-liquified or liquified in the solution--always requires temperatures below -40.degree. C. under normal pressure. To attain and maintain these low temperatures, elaborate cooling units and special insulating measures are required. The energy consumption for this type of cooling is high. Beyond that, the low temperatures and the great temperature fluctuations lead to rapid fatigue of the equipment employed, and the reaction vessels and associated equipment have only a short service life.